<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .dahezi {
            position: absolute;
            top: 200px;
            left: 50%;
            height: 200px;
            width: 200px;
            background-color: blue;
            margin-left: -100px;
        }

        .sanjiao {
            position: relative;
            top: -40px;
            left: 50%;
            margin-left: -20px;
            height: 0;
            width: 0;
            line-height: 0;
            font-size: 0;
            border: 20px solid transparent;
            border-bottom-color: blue;
        }
    </style>
</head>

<body>
    <div class="dahezi">
        <div class="sanjiao">

        </div>
    </div>

</body>

</html>